The present invention relates to an apparatus for heating a sample, such as chemical reaction mixtures, whose dielectric properties varies during the heating process. In particular, the present invention relates to a microwave heating apparatus comprising a resonant cavity in which the resonance conditions and the coupling factor of radiation to the cavity are easily adjustable. The resonance conditions and the coupling factor can be adjusted in response to the dielectric properties of the sample in order to optimise the amount of absorbed power and thereby obtain control of the sample heating process.
One of the major obstacles for an organic chemist today is the time consuming search for efficient routes in organic synthesis. The challenges for the pharmaceutical industries and the organic chemist include identification of ways of reducing time in drug development, identification of ways of creating chemical diversity, development of new synthesis routes and maybe reintroduction of old xe2x80x9cimpossiblexe2x80x9d synthetic routes. Also, it is a constant challenge to reach classes of totally new chemical entities.
Chemical reactions are often performed at elevated temperature to enhance the speed of the reaction or supply enough energy to initiate and maintain a reaction. Microwaves assisted chemistry offers a way to perform reaction processes and circumventing at least some of the above-mentioned problems, namely
speeding up the reaction time with several orders of magnitudes,
improving the yield of chemical reactions,
offering higher purity of the resulting product due to rapid heating and thereby avoiding impurities from side reactions, and
performing reactions that are not possible with conventional thermal heating techniques.
Recent developments have lead towards apparatuses comprising a microwave generator, a separate applicator for holding the sample to be treated, and a waveguide leading the generated microwave radiation from the generator and coupling it into the applicator. Even if the system consists of a 2450 MHz, TE10 waveguide to which a magnetron generator is connected in one end and the sample container is in the other end, there is a need for a matching device in the form of at least a metal post or iris between the generator and load, in order to achieve a reasonable efficiency.
When coupling electromagnetic radiation such as microwaves from a source to an applicator, it is important to match the waveguide impedance and the applicator impedance (with sample) in order to achieve a good transfer of power. However, the dielectric properties of the sample will influence drastically upon the impedance of the applicator, as well as its electrical size, and the dielectric properties of the sample often change considerably with both temperature and applied frequency. Thus, an impedance mismatch between the source and the applicator will often occur and the coupling and thereby the heating process becomes less efficient and difficult to predict
U.S. Pat. No. 5,837,978 discloses a microwave heating system applying a resonant multimode applicator comprising means for impedance matching during a heating process in order to achieve resonance of the system. The matching or tuning is carried out by adjusting the height of the applicator and the position of a microwave antenna/probe in the applicator (see e.g. column 7, lines 17-24 or column 8, lines 33-39).
In multimode cavities, the electric field is a superposition of several longitudinal modes and several transverse modes. When a multimode applicator is tuned to resonance, one changes the balance between these modes and thereby the spatial energy distribution. The energy distribution is therefore neither spatially uniform nor constant during the heating process, which makes it difficult to obtain reproducible results since a small change of the position or size of the sample, or a resonance tuning (performed by the user or by a change in the dielectric properties of the sample), will resultin different power absorption. Rotation of the sample in the oven does not significantly improve the reproducibility, since some of the modes, as a matter of fact most of the modes in a true multimode system, have a tendency to heat the outer parts of the sample more strongly. This leads to a position dependent heating of the sample, which is also dependent upon the resonance tuning. The samples used in microwave chemistry typically have volumes ranging from a few xcexcL to xcx9c10 mL, and it is therefore crucial to have a uniform and known energy distribution.
WO 99/17588 discloses a microwave oven having a conductive member for controlling the feeding of microwave power from a waveguide to a multimode applicator. The conductive member acts as a diffracting resonator and provides a local region with a particular field pattern. When the member is rotated, the field changes, giving rise to an advantageous feeding of microwave power to the multimode applicator. The conductive member is preferably an elliptic ring member.
EP 552 807 A1 discloses a similar microwave oven having a rotatable metal reflector in a waveguide for impedance matching between the waveguide and a heating chamber.
Single mode applicator resonators offer a possibility of high field intensities, high efficiency and uniform energy distributions. The use of single mode applicators have been reported, see e.g. U.S. Pat. No. 5,393,492 and U.S. Pat. No. 4,681,740. However, since the dielectric properties of the sample changes the resonance frequency and since magnetrons usually only provide a fixed frequency or only a minor adjustment around the centre frequency of the magnetron, the generated frequency and the resonance frequency of the mode will detune as the sample heats. Thereby the high intensity in the resonant mode is lost.
U.S. Pat. No. 2,427,100 and NL Octrooi No. 75431 both discloses means for adjusting the point impedance, or wave reflection, in microwave waveguide transmission systems by having a conducting deflector rotatably mounted in the waveguide. Both systems tune the waveguide system by introducing a reactance into the waveguide. Note that only the scattering, i.e. reflection of a specific waveguide mode, is affected.
U.S. Pat. No. 4,777,336 discloses a method for controlling heating patterns in single or multimode applicators by tuning the applicator using a probe or sliding shorting plates within the applicator.
It is generally a disadvantage of the multimode applicators that the spatial energy distribution changes when it is tuned for impedance matching.
It is another disadvantage of the multimode applicators that the applicator has a non-uniform energy distribution.
It is a further disadvantage of the multimode applicators that the multimode heating pattern is not reproducible (i.e. very sensitive to its dimensions) and may change as a function of the temperature of the load.
It is a disadvantage of the prior art single mode applicator apparatuses that there are no efficient and durable means for tuning the resonance frequency in response to the dielectric properties of the load, since galvanic contacting by for example screw posts or metal vanes is needed for efficient control of also small coupling factors and the air distances to the waveguide walls tend to become so small that there is a risk of arcing.
In view of the foregoing, an object of the present invention is to provide a microwave heating apparatus wherein the samples can be uniformly heated by using a single mode applicator.
Another object of the present invention is to provide a microwave heating apparatus that has a high efficiency in that the coupling of radiation to a sample held in the applicator is improved.
Still another object of the present invention is to provide a microwave heating apparatus wherein coupling to a single mode applicator and a resonance frequency of the applicator can be adjusted in response to variations in dielectric properties of a sample in the applicator using a single rotatable deflector.
In a first aspect, the present invention provides a heating apparatus comprising:
generating means for generating electromagnetic radiation at a wavelength,
a waveguide for guiding the generated electromagnetic radiation to a waveguide applicator for holding a sample to be heated, the sample having dielectric properties xcex5sample which varies as a function of a temperature of the sample, the waveguide and the waveguide applicator supporting a single normal transverse mode,
a deflector formed by a closed loop defining a plane, said deflector having a resonance frequency xcexddefl and a thickness in the interval [xcex/30; xcex/5] in a direction normal to said plane, the deflector being rotatable around an axis being at least substantially parallel to said plane,
the deflector being positioned in the waveguide so as to form a resonant cavity with the sample and the waveguide applicator, said cavity having at least one resonance frequency xcexdcav being dependent upon at least xcex5sample, xcexddefl, and an angle of rotation of the deflector, xcex1defl.
In the present context, waveguide should be interpreted as any means capable of guiding electromagnetic waves such as electromagnetic radiation. The waveguide may be a waveguide in the form of metallic channels for guiding waves such as radiation or cables such as coaxial cables for guiding waves such as electrical signals. The waveguide may also comprise active and/or passive components such as couplers, dividers, splitters, combiners, circulators, power meters, artificial samples, spectrum analysers etc.
The waveguide may typically support only a single transverse mode, TE or TM, depending upon its design. The waveguide is preferably connected to the applicator so as to transfer energy from modes in the waveguide to modes in the applicator. In order for the coupling to be efficient, the impedance of the waveguide must be at least substantially matched with the impedance of the applicator, and there may also be a field matching (i.e. possibility of continuous energy transfer by field similarities in the two guides). The coupling of radiation, and hence of energy, from modes in the waveguide to modes in the applicator can, under conditions of field matching, be quantified by the coupling factor defined as the ratio between the impedance of the waveguide and the impedance of the applicator. It is typically desirable to have as good an impedance matching as possible (or equivalently, a coupling factor as close to 1 as possible) under the actual conditions. This impedance matching (or coupling factor optimisation) may be obtained under different conditions depending on different parameters such as the absorbency of the sample and the design of the system. When rotating the deflector for adjusting the coupling factor, one may also adjust the resonance frequency of the cavity xcexdcav. However, and as will be shown later, the optimisation of the coupling factor need not be coincident with the tuning of xcexdcav to equal the generated frequency. In a preferred embodiment, both the waveguide and the waveguide applicator preferably supports a TE10 mode so that the condition of field matching is fulfilled.
A waveguide applicator is in its simplest form a waveguide terminated by e.g. a short circuit wall, an iris or equivalent, which is adapted to hold a sample to which the microwaves are applied. Thus a waveguide applicator supports the same TE or TM mode as the waveguide of which it is an end-part. Depending on the waveguide and the mode in the waveguide, the applicator need not have the exact same cross-sectional dimensions as the waveguide. Typically, the waveguide supports a TE10 mode wherein the electric field has no variations in the vertical direction, hence, in this case only the horizontal dimension (the width) of the waveguide and the waveguide applicator needs to be at least substantially equal. The geometrical constraints between the waveguide and the waveguide applicator for different designs will be obvious for the person skilled in the art bearing the need for field matching in mind.
A single mode applicator is an applicator comprising an applicator cavity adapted to support only a single resonant mode within the frequency spectrum of the applied radiation. Hence, a waveguide applicator is also single mode applicator, and depending on the context, the waveguide applicator may also be denoted a single mode applicator, or simply an applicator.
In order to reach high a high field strength within the applicator, it is preferable that the resonance frequency of the cavity is close to or substantially equal to the frequency corresponding to an amplitude maximum in the generated frequency spectrum. The resonance conditions can be expressed either as a tuning of the reactive impedance (the capacitative and inductive reactance) of the applicator, or as an adaptation of the electrical length of the applicator to make it equal to xcex/2, where xcex is the wavelength of the applied radiation.
The electric length is a measure of the distance traversed by electromagnetic radiation in a medium in time t, and is approximately equal to the corresponding distance electromagnetic radiation would have traversed in vacuum in the same time t If e.g. a high permittivity medium of length x is inserted in a radiation path, the electrical path length is increased by (nxe2x88x921)x, where n is the refractive index of the medium.
According to the present invention, the deflector is formed by a closed loop defining a plane. In this plane, the deflector has a width xe2x80x9caxe2x80x9d and a height xe2x80x9cbxe2x80x9d. Also in this plane, the material forming the loop has a radial thickness xe2x80x9ccxe2x80x9d. The deflector has an axial thickness xe2x80x9chxe2x80x9d along an axis normal to the plane of the deflector. The circumference of an inner perimeter of the closed loop of the deflector determines the inherent resonance frequency xcexddefl of the deflector, and by that the frequency of maximal blocking, when it is placed with its plane perpendicular to the direction of power flow in a waveguide. The deflector may be rotated so as to have its plane perpendicular to (or its axis parallel with) the waveguide where it will efficiently reflect radiation having a frequency equal or close to xcexddefl (blocking position). Also, the deflector may be rotated to a position where its plane is parallel with (or its axis is perpendicular to) the waveguide, where it will only reflect radiation comparable to that of a plate of conducting material having the same profile (open position). In between these positions, the deflector can be characterised by a complex reflection coefficient R(xcexd, xcex1defl) depending on the frequency and angle of rotation. Hence xcexddefl and xcex1defl at least partly determine the coupling of radiation between the waveguide and the waveguide applicator. The phase of the complex reflection coefficient varies as a function of the angle of rotation of the deflector. This may be interpreted as that the position of the minimum of the standing (reflected) wave varies with the angle of rotation thereby introducing a phase delay or shift as the deflector is rotated.
As stated previously, the deflector forms a resonant cavity with the waveguide applicator (with sample). As said above, the deflector may affect the electrical distance for at least part of the electromagnetic waves guided towards the applicator so as to virtually change the effective length of the cavity. Since this effect depends in the angle of rotation of the deflector, the resonance frequency of the deflector may be tuned by rotating the deflector.
Since the resonant frequency of the cavity may change when the permittivity of the sample varies, the deflector action may compensate this change, thus keeping the resonant frequency substantially constant and thereby provide a possibility to provide a high microwave heating efficiency.
The complex reflection coefficient of the deflector, the resonance frequency xcexdcav of the cavity, and the coupling of radiation between the waveguide and the cavity are closely related. For illustrative purposes, the tuning of dimensions and the angle of rotation of the deflector may be considered as a balance between coupling radiation to the cavity and keeping the coupled power in the cavity. If for example xcexddefl=xcexdcav, the deflector in its blocking position may form a very efficient xe2x80x9cend mirrorxe2x80x9d for resonant radiation in the cavity, however, only very little radiation (having the right frequency xcexdcav) may be coupled to the cavity. When the deflector is rotated towards its open position, more radiation may be coupled to the cavity, but on the other hand, the deflector may not form a very efficient xe2x80x9cend mirrorxe2x80x9d, and more power may be lost from the applicator. Thus at some position between blocking and open position, a maximum in the power in the cavity may be expected. If on the other hand xcexddefl is very different from xcexdcav, radiation having a frequency xcexdcav may efficiently be coupled to the cavity even when the deflector is in its blocking position, but the deflector may not form a very efficient xe2x80x9cend mirrorxe2x80x9d. Hence, and a maximum in the power in the cavity may be expected at a xcexddefl which is not equal to but neither too different from xcexdcav.
A proper choice of the axial thickness significantly larger than the radial thickness will provide a desirable location change of the phase of the reflected wave when the deflector is rotated. Preferably, the axial thickness of the deflector is in the interval [xcex/20;xcex/10], such as within the interval 3 to 25 mm in a 2450 MHz, TE10 waveguide with dimensions 86xc3x9743 mm (width x height). For waveguides with lower heights, such as 25 mm, the axial thickness must be smaller; a suitable dimension has been found to be about 10 mm. Also in a preferred embodiment, the radial thickness of the deflector is between 0.1 mm and 5 mm.
Preferably, the deflector is shaped like an ellipse having a major principal axis of length a and a minor principal axis of length b. Alternatively, the deflector is shaped like a trapezium, such as a rectangle having a width a and a height b. The choice of the detailed shape of the closed loop depends on the desired xe2x80x9cleakage propertiesxe2x80x9d, where an elliptical shape may give maximum blocking according to the prior art.
For a predetermined set of conditions such as sample volume, sample permittivity, position of the sample in the applicator, and coupling of the guided waves between the waveguide and the applicator, the applicator may become anti-resonant. In this case, the resonance frequency of the applicator and/or the coupling of the guided waves between the waveguide and the applicator may be adjusted by comprising a member of a material having a relative permittivity larger than 5, such as larger than 10, preferably larger than 25 positioned within the applicator. In order to prepare relative permittivity of the material, it may comprise ceramic materials comprising one or more materials selected from the group consisting of Al2O3, TiO2 or XTiO3, where X is any group II element such as Ca or Mg. The relative permittivity and/or the shape and/or the size of said member might be chosen so as to make the applicator resonant at said predetermined set of conditions.
Optionally, the apparatus may further comprise means for adjusting the position of the sample in the applicator in order to adjust the effect of the sample upon the resonance frequency of the cavity and/or the coupling of the guided waves between the waveguide and the applicator. Preferably, the means for adjusting the position of the sample comprises means for adjusting a substantially vertical position of said supporting means.
In order to reduce the amount of scattered waves towards the generator, the apparatus may further comprise a first circulator and a first dummy load, wherein the first circulator is adapted to deflect at least part of electromagnetic waves reflected from the applicator towards the first dummy load. One or more power measuring means may be positioned so as to measure the power of at least part of the electromagnetic waves deflected by the first circulator. The one or more power measuring means is preferably operationally connected to a first memory means for storing the measured power.
The generator may comprise a magnetron or a semiconductor based generator and a semiconductor based amplifier. The semiconductor-based amplifier preferably comprises one or more silicon-carbide power transistors. Alternatively, the generator may comprise both a magnetron and a semiconductor based generator.
The sample is preferably held in a container which is substantially hermetically closed and adapted to withstand pressure.
Also, it is often of interest to monitor the temperature of the sample during heating. For this purpose, the apparatus may comprise a thermal radiation sensitive element adapted to determine a temperature of the sample and positioned so as to receive thermal radiation emanating from the sample.
Both the high pressures and the high temperatures of the sample imply a risk for the container to break and thereby leak sample in the applicator. The breaking of the container can be such as an explosion or simply a melting of the container. In order to protect the deflector and the waveguide in case of breaking of the container, the apparatus may comprise a screen for separating the deflector and the waveguide from the container so as. The screen is preferably substantially transparent to the electromagnetic waves guided towards the applicator, and may comprise one or more of the materials selected from the group consisting of: PTFE (Teflon(copyright)) TPX, polypropene or polyphenylidenesulphide (PPS, Ryton(copyright)). Optionally, the applicator also comprises a drain for draining sample from within the applicator. Preferably, the drain leads to a receptor for receiving the sample drained from the applicator.
The apparatus may be further automated by comprising means for placing the sample within the applicator. If the sample is loaded into the container outside the apparatus, the placing means is means for placing the container at least partly within the applicator.
In order to allow for a larger variation in the power and/or frequency of the generated waves, the apparatus may further comprise a second generating means for generating electromagnetic waves. In this case the waveguide is adapted to guide at least part of the electromagnetic waves generated by the first and second generating means to the applicator. In order to allow parallel processing of samples, the apparatus may further comprise a second applicator for holding a container holding a second sample. In this case the waveguide is adapted to guide at least part of the electromagnetic waves to the first and second applicator. The second applicator may also comprise all the features described in relation to the applicator above. The combination of two or more generators and two or more applicators allows for a large system wherein the generated power is dosed to each applicator individually.
The term microwave is intended to mean electromagnetic radiation in the frequency range 300 MHz-300 GHz. Preferably, the apparatus and methods according to the invention are performed within the frequency range of 500 MHz-300 GHz, preferably within the frequency range 500 MHz-30 GHz such as 500 MHz-10 GHz such as 2-30 GHz such as 300 MHz-4 GHz such as 2-20 GHz such as 0,5-3 GHz or within the range 50-100 GHz.
In the present context, the term xe2x80x9capparatusxe2x80x9d designates one or several pieces of equipment which, as a whole, comprise the parts, means and elements that characterise the invention. Accordingly, the apparatus may appear as a distributed system where individual parts or means are not located in close physical proximity to each other. As an example of this architecture, the memory means may be physically located on e.g. a personal computer (PC) while all the mechanical parts may appear as a joint unit.
In a second aspect, the present invention provides a method for applying the apparatus of the first aspect. Thus, according to the second aspect, the present invention provides a method for heating a sample, said method comprising the steps of:
I. providing a heating apparatus according to the first aspect, and inserting the sample in the applicator,
II. generating electromagnetic radiation at a first output power level,
Ill. rotating the deflector for adjusting the coupling factor between the waveguide and the resonant cavity.
When a heating process is initiated, the sample has a first temperature T1. The method preferably further comprises the steps of:
heating the sample to obtain a second temperature T2 greater than T1,
rotating the deflector for adjusting the coupling factor between the waveguide and the resonant cavity in response to the variation in the dielectric properties xcex5sample of the sample.
The above steps may be repeated several times during a heating process.
The present invention allows for designing and/or optimising of a heating process of a sample. Thus, the method according to the second aspect may further comprise the steps of:
IV. performing the following steps one or more times:
positioning the deflector in a first position and measuring a first power of electromagnetic radiation reflected from the waveguide applicator, the reflected radiation corresponding to said first position of the deflector,
rotating the deflector to a second position that is different from the first position and measuring a second power of electromagnetic radiation reflected from the waveguide applicator, the reflected radiation corresponding to said second position of the deflector, and
V. determining a preferred position of the deflector based on the amount of power reflected from the waveguide applicator in at least the first and second position.
These measured powers are preferably inversely proportional to the power absorbed in the sample at the first and second position of the deflector. Preferably, this designing and/or optimising are only performed once for each type of sample or reaction since the obtained parameters can be saved for later use. Hence, the method may further comprise the steps of:
VI. providing a first storing means,
VII. storing information relating to the first position in the storing means and storing the measured first power in relation thereto, and
VIII. storing information relating to the second position in the storing means and storing the measured second power in relation thereto.
It will often be of interest to store measured powers corresponding to a plurality of different positions, and the steps IV, VII, and VIII may be repeated as often as desired. The deflector angles and the powers may be stored as a listing such as a table, in the storing means. According to the second aspect, step V may comprise processing of the stored measured powers for determining the preferred position of the deflector corresponding to a local or an absolute minimum in the measured power, or to a predetermined ratio of the measured power to the first output power level.
After the determination of a preferred position of the deflector, the method may further comprise the steps of positioning the deflector in he preferred position in order to heat the sample. Optionally, the method also comprises the step of, after having positioned the deflector in the preferred position, generating electromagnetic radiation at a second output power level which is larger than the first output power level in order to heat the sample at a higher rate.
By comparing the stored measured powers with corresponding stored measured powers measured for a different second sample, it is possible to determine a measure of the relative permittivity of a first sample relative to the relative permittivity of the second sample.
Alternatively, by comparing the stored measured powers with corresponding stored measured powers measured for a second sample of known chemical composition, it is possible to determine an indication of the chemical composition of the first sample relative to the chemical composition of the second sample. If the first sample comprises at least one reactant for performing a chemical reaction, the method may further comprise the steps of:
performing the chemical reaction with the at least one reactant, and
determining a degree of reaction for the chemical reaction using the indication of the chemical composition of the sample,
where the degree of reaction is a measure of the extent to which the reactants has reacted to form products in a chemical reaction.